Why is it difficult to make a good Superman movie?

I'd argue that Superman and Superman II are actually quite good. They're dated in terms of production value, though at the time the effects were much better than anything previous. I really did believe a man was flying. I'd also put Superman Returns in a category other than "especially bad." I liked it more than I liked Superman III.

What I think makes the earlier films work is that they were made in an era that was not too different than the era in which Superman was created. He's a product of his times, and much of what makes him iconic still existed in 1980, but doesn't in 2018. He's a glasses-wearing square from a time when glasses and being a square weren't status symbols. He wears a suit, tie, and fedora, works as a newspaper reporter, and changes into his costume in a phone booth. None of the five things mentioned in that sentence have any meaning to anyone under 25, except perhaps a fedora, which no longer holds the same connotation today as in the past.

I suspect that a Superman film set in, say, 1960, that remained small in its scale would be success. The original film was good because of the actors and the script, not because of any massive battles. I don't even think there was a super-villain, just Gene Hackman. The sequel was fun because Superman was pitted against other super-beings, but even there the effects were small, and the scale limited. I'd argue Superman saving the plane at the start of Superman Returns was more emotional and exciting than anything in either Snyder film.

Instead of Superman and 5 other heroes waging cataclysmic CGI wars against demons from another dimension, find a great cast, write a fun script with good dialogue, and have some exciting save-the-day moments.

As an aside, I'd say one big mistake the makers of the Superman films are making is trying too hard to copy the Marvel films. Superman is best on his own, and the scope of his powers don't mesh that of Batman, at least not cinematically. Comic books are very different animals, and it's fine there, but in the Batman universe no one is superhuman, so the existence of a god-like alien throws everything out of whack. Nolan's films worked well because they didn't venture into the supernatural.

Okay, I'll take a stab at this since I have about 45 years of Superman knowledge (hardly an expert though).

The problem with Superman (comics, novels, TV and film - oddly animation and radio tend to do okay) is one of inconsistency. Over the decades, they keep increasing his power levels without reason and then decreasing them without reason. The problem is, they keep ramping him up. It gets to the point where it becomes extremely difficult to find villains for him to fight. That's one point and I'll give you some examples from the movies.

1) In Superman, he travels fast enough to go back in time. That in itself robs every other film of suspense because he can always call a do-over to any problem.
2) In Superman 2, we see it takes him several minutes to get from Metropolis to Paris. With him knowing the terrorists have a nuke. But....we saw him go fast enough to time travel last movie!
3) Lex as a villain becomes problematic because it's SUPPOSED to be brains vs. brawn. Luthors' intellect making him a match for Supermans' powers. Except, we hear Jor-El tell Kal (and us) that Superman is getting the knowledge of 28 known galaxies. So, um....what? Luthor is smarter than THAT?
4) The invented hand beams and telekinesis we see in Superman 2. WTF? Like there's not enough in his stable, you need to add stuff?

The other problem with Superman is how you want to portray him. Donner took a unique idea, that he was among us but never one of us and ran with it. So much that comics adopted this. And everything became about him being an alien. It's hard to remember but for decades, Superman was okay with his status and was not all morose about it. He could've just been a guy who got these powers in a lab accident for all he cared. But because Batman showed us readers liked a little sadness in their heroes, everyone got a touch of it in their origins. Barry lost his mom, Hal lost his dad. But here's the problem (IMO)....he's Superman. Life does not suck for him.

It's not because Superman is to strong, because has plenty of foes that can keep up with him, or because he is to nice, it's because the directors didn't had the right vision for him.

Superman Returns suffered from trying to be to much like the Reeves films, and there was no real adversary for him. Man of Steel made the character unrecognisable (seriously a dark Superman?) and in BvS people didn't cared for the way he was presented. The guy had almost no lines and the Martha scene was horrible.

With a good director we would get a fantastic Superman I'm sure.

The original Christopher Reeve films were good.

The problem with the Brandon Routh one was that tried to follow on from earlier Super-Man movies, without outright stating that's what they were doing. There were good bits in it. The plane scene early on felt like a classic Superman scene. But it just felt a bit of a mes. Was it a sequel? A brand new film? A bit of both? I don't think the problem was Routh himself.

And I don't hate the new movies with Henry Cavill, but one problem is they tried to make Superman more edgy. Make him dark/brooding. The character wasn't traditionally like that. He felt like Batman with powers. This was most of a problem in Batman vs Superman, because the characters were supposed to be quite different in how they dealt with things. There wasn't that contrast between the two. They'd toned that back a bit with Justice League, in mu opinion.

Though, having said all that, I think the OP's assessment of Superman being a bit too goody goody and overpowered does on occasion make him hard to write for. But that's not really an excuse, because there's decades worth of comics, cartoons, movies and TV shows of Superman. Not all stories are going to be bad.

I think they should've made a Superman vs Doomsday movie, rather than shoe-horn that storyline into Batman vs Superman. It felt rushed. If that'd come after Batman vs Superman or even after Justice League, it would've been better. The original comic storyline had a lot of the JLS in as well, with Superman being called in as the sort of saviour. It'd have been more faithful to the comics then, and the JLS' reaction to it would've added to the movie. What you got instead was Batman vs Superman movie with a guest cameo by Doomsday.